1. Field of the Invention
The present invention relates to a method of controlling turn-on of light source and an image forming apparatus, and particularly to a method of controlling turn-on of a light source for an image forming apparatus in which a laser beam emitted from the light source is scanned on an image carrier by rotating a rotational polygonal mirror to form an image on the image carrier, and an image forming apparatus using the control method.
2. Description of the Related Art
Image recording apparatuses for recording an image with a laser beam, such as a laser printer, an electrophotographic copying machine, etc. have propagated. In these image recording apparatuses, a scanning operation using a laser beam is carried out on a photosensitive medium by an optical scanning device.
In the optical scanning device, generally, a laser beam output from a semiconductor laser is modulated on the basis of image data, and then made incident through a collimator lens, etc. to a reflection surface of a rotational polygonal mirror (hereinafter referred to as xe2x80x9cpolygon mirrorxe2x80x9d) rotating at a predetermined speed. By the rotation of the polygon mirror, the laser beam is deflected while the incidence angle of the laser beam is continuously varied, thereby performing a fast scanning operation on the photosensitive medium with the laser beam. The laser beam reflected from the reflection surface of the polygon mirror is guided through an fxcex8 lens, a cylinder mirror (or cylinder lens) or the like to the photosensitive medium to scan the photosensitive medium at a constant speed and also to be focused onto the photosensitive medium, whereby an image is exposed and recorded on the photosensitive medium.
The optical scanning device is provided with a start-of-scan (SOS) sensor at the substantially equivalent position as the start-of-scan position of the photosensitive medium, and the laser beam at the start-of-scan position is guided to the start-of-scan sensor by a reflection mirror to obtain a start-of-scan (SOS) signal. An exposure recording (image writing) timing, a laser diode light amount control (APC: Auto Power Control) timing, etc. are determined on the basis of the start-of-scan signal.
Here, when the beam width of the scan direction of the laser beam incident on the polygon mirror is set to be sufficiently larger than the size of the reflection surface of the polygon mirror (in the case of a so-called over-field type optical scanning device), the scanning operation is carried out so that the laser beam incident on the polygonal mirror is cut out by the polygonal mirror, and thus the ratio of the width of an image area to the scan-permissible width that is, the effective scanning rate can be set to a sufficiently large value.
This enables the frequency of the video clock to be reduced when the same resolution is required to be obtained at the same process speed, and also the cost of ASIC of the image processing to be reduced. Further, the radiation noise and the heating can be reduced, so that the cost required for shield design, etc. can be reduced.
However, in the case of an image forming apparatus having a function of varying the magnification in the fast scanning direction of the image by varying the frequency of image clocks, there is a case where a sufficient processing time cannot be secured to perform processing such as light amount control processing for a laser diode or the like when an optical scanning device having a large effective scanning rate such as an over-field type is used.
Next, there will be described the output timing of each of an SOS signal, an image area signal (LS signal), an SOS pre-turn-on signal and an APC signal, that is, the output timing of each signal on the basis of the output timing of the SOS signal (i.e., the lapse time from the output time of the SOS signal to the output time of each signal) and the number of image clocks when the magnification in the fast scanning direction of the image is not changed (nominal state) are respectively set as shown in Table 1.
Table 1 relates to the case that the outputting timing of a next SOS signal on the basis of the detecting time of the SOS signal (i.e., the timing at which the next SOS signal is detected after the previous SOS signal is detected: SOS period) is set to 350 xcexcs (microsecond).
Here, for example when the frequency of the image clocks is reduced by 2% in order to increase the magnification in the fast scanning direction of the image, the above values are set as shown in the following Table 2. Since the scanning speed is unvaried, the SOS period is kept to 350 xcexcs.
According to this table, the output timing of the next SOS signal is set during the execution of the light amount control due to the output of the APS signal. Therefore, there may occur such a case that the light amount control of the laser beam is inaccurate or the detection of the laser beam by the SOS sensor cannot be accurately performed and thus no SOS signal is output.
Therefore, Japanese Laid-open Patent Application No. 268332/1999 discloses a technique of accurately performing the light amount control of the laser diode, etc. even when the frequency of the image clocks is made variable in order to vary the magnification in the fast scanning direction of the image.
This technique is characterized in that an image forming apparatus is provided with a controller for controlling the output timing of each of a turn-on indicating signal for indicating turn-on of a laser beam (SOS pre-turn-on signal) and a light amount control indicating signal for indicating execution of light amount control of the laser beam (APC signal) so that these signals are generated at a fixed timing irrespective of the frequency of the image clocks.
More specifically, when the output finish timing of the LS signal exceeds the output start timing of the APC signal under the nominal state, the output start timing of the APC signal is altered just after the SOS signal is output, and also the output start timing of the SOS pre-turn-on signal is calculated on the basis of the following equation (1):
Timing after alteration=timing before alterationxc3x97(100xe2x88x92magnification to be varied)/100xe2x80x83xe2x80x83(1)
Accordingly, the SOS pre-turn-on signal is output at a fixed timing at all times, and the detection of the laser beam by the SOS sensor can be accurately performed. For example, when the frequency of the image clocks is reduced from that under the nominal state of Table 1 by 2% in order to increase the magnification in the fast scanning direction of the image, the output timing of each signal is set as shown in the following Table 3.
As described above, even when the frequency of the image clocks is made variable in order to alter the magnification in the fast scanning direction of the image, the time required to perform the light amount control of the laser beam can be sufficiently secured, and the light amount control can be accurately performed. In addition, the timing of the SOS pre-turn-on signal can be sufficiently secured, and the detection of the laser beam by the SOS sensor can be accurately performed.
In connection with a recent coloring requirement, an image forming apparatus having a function of forming (printing) a color image has rapidly propagated. The formation of the color image can be implemented by superposing four colors of cyan (C), magenta (M), yellow (Y) serving as three primary colors and black (K) on a photosensitive drum. However, it needs a longer processing time than formation of a monochromatic (white and black) image, and has lower productivity. Therefore, there has been proposed a tandem type multi-color image forming apparatus which is equipped with a photosensitive drum and a light scanning device for each color of C, M, Y, K.
In the tandem type multi-color image forming apparatus, after a laser beam modulated on the basis of each color image data is generally output from the light scanning device of each corresponding color of C, M, Y, K to expose the corresponding photosensitive drum to the laser beam and form the corresponding latent image on the photosensitive drum, the respective latent images on the photosensitive drums are developed to form corresponding visible images on the photosensitive drums, and then the visible images are transferred onto the same recording medium while superposed on one another. That is, the respective images are formed at the same time, and thus the productivity can be greatly enhanced.
In the tandem type multi-color image forming apparatus, if the respective color images are subtly displaced in writing position from one another, it appears as color displacement when the respective images are superposed and then transferred onto a recording medium, resulting in reduction of the image quality of an image thus formed. In view of this problem, Japanese Patent Application No. 46468/1999 proposes a technique of correcting the color displacement in the slow scanning direction in the tandem type image forming apparatus.
More specifically, in the optical scanning device provided every color, a polygon motor for rotating a polygon mirror provided in the optical scanning device is subjected to PLL (phase locked loop) control, thereby rotating the polygon mirror at a fixed speed. More specifically, the comparison clock corresponding to the rotation speed of the motor concerned is obtained, and the PLL control is performed so that the comparison clock and the reference clock of a predetermined frequency are locked with keeping a predetermined phase difference.
At this time, in order to perform the image forming processing with high precision, the SOS signal is used as the comparison clock. However, in order to increase the lifetime of the laser diode and reduce the time required to start the polygon motor, a pulse signal (hereinafter referred to as xe2x80x9cFG signalxe2x80x9d) synchronized with the rotational speed of the polygon motor from an FG sensor for detecting the number of revolutions of the polygon mirror is used as the comparison clock when the image forming processing is not carried out (on standby for the image forming processing), and it is changed the comparison clock to the SOS signal from the SOS sensor when the image forming processing is carried out.
When the color displacement occurs in the slow scanning direction, in order to correct the color displacement concerned, the frequency of the reference clock of a color for which an image writing position should be corrected (hereinafter referred to as xe2x80x9ccorrection colorxe2x80x9d) is changed, and it is returned to the original frequency after a predetermined time elapses. With this operation, the polygon motor for rotating the polygon mirror in the optical scanning device for the correction color is subjected to the PLL control on the basis of the frequency of the reference clock thus altered, thereby changing the rotational speed.
Thereafter, upon returning of the reference clock to the original frequency, the rotational speed is returned to the original rotational speed again. At this time, the rotational phase of the polygon mirror is varied by the amount corresponding to the variation of the frequency. Accordingly, the position of the slow scanning direction of the image of the correction color concerned can be relatively displaced by the phase difference in the slow scanning direction of the images of the other colors, thereby offsetting the color displacement from the images of the other colors.
However, in the case where the comparison clock is switched between the FG signal and the SOS signal in the PLL control operation of the polygon motor, although the polygon motor is rotated at a predetermined rotational speed (at a predetermined number of revolutions) when the switching operation is carried out, it may be judged that the number of revolutions is not the predetermined number of revolutions if there is a phase difference between these two signals. By the action of the PLL control, the rotational speed of the polygon motor is varied in accordance with the phase difference between the FG signal and the SOS signal at this time (the number of revolutions is varied).
For example, if the comparison clock is switched from the FG clock to the SOS signal at a timing indicated by an arrow A when the FG signal and the SOS signal have the same frequency and the same duty as shown in FIG. 11, the period of the comparison clock is instantaneously lengthened. Therefore, it is judged in the PLL control that the number of revolutions of the polygon motor is reduced, and thus the speed of the polygon motor is increased. Through this control, the number of revolutions of the polygon motor is varied as shown in FIG. 12. Due to the variation of the rotation, the detection timing of SOS is earlier than the turn-on (SOS pre-turn-on) for detecting the SOS signal, and thus there may occur such a case that the SOS signal is missing.
Further, when a monolithic type laser diode having plural light emission points is used, each of the plural light emission points must be independently turned on to perform the light amount control because only one photodiode (PD) for detecting the output light amount is provided. That is, any one of the light emission points is turned on during the APC period. On the other hand, it is necessary to turn on the plural light emission points at the same time in order to detect the SOS signal, and thus no SOS signal may be detected due to an insufficient light amount when the detection timing of the SOS signal and the APC period are overlapped with each other.
Here, a case where a monolithic type laser diode having two light emission points is used will be described in detail.
FIG. 13 is a timing chart for each signal when the light amount control and the SOS pre-turn-on under the nominal state shown in Table 1 are carried out. In FIG. 13, xe2x80x9cSOSxe2x80x9d represents the detection timing of the SOS signal, xe2x80x9cAPC-Axe2x80x9d represents an APC signal which instructs execution of the light amount control of one light emission point, xe2x80x9cAPC-Bxe2x80x9d represents an APC signal which instructs execution of the light amount control of the other light emission point, xe2x80x9cVDATA-Axe2x80x9d represents a turn-on signal which instructs turn-on of one light emission point, xe2x80x9cVDATA-Bxe2x80x9d represents a turn-on signal which instructs turn-on of the other light emission point, and xe2x80x9cemission light amountxe2x80x9d represents the emission light amount from the laser diode (the total emission light amount from the two light emission points).
As shown in FIG. 13, during the APC period, the APC signal of the one light emission point is set to H (High) level (see APC-A), and the APC signal of the other light emission point is set to L (Low) level (see APC-B), and the light amount control (APC) is carried out by turning on the one light emission point (see VDATA-A) and turning out the other light emission point (see VDATA-B). On the other hand, during the turn-on period (SOS pre-turn-on) for detecting the SOS signal, both the light emission points are turned on (see VDATA-A, VDATA-B).
When the polygon motor is increased in speed by switching the comparison clock from the FG clock to the SOS signal or the like under the control as shown in FIG. 13, the period of the detecting timing of the SOS signal (hereinafter referred to as xe2x80x9cSOS periodxe2x80x9d) is shortened. FIG. 14 is a timing chart for each signal when the number of revolutions of the polygon motor is varied by about 1.5%.
As shown in FIG. 14, when the number of revolutions of the polygon motor is varied by about 1.5%, the SOS period which is equal to 350 xcexcs in FIG. 13 is reduced by 5.25 xcexcs (35 xcexcsxc3x970.015) and thus equal to 344.5 xcexcs. Therefore, the SOS detection timing is earlier than the SOS pre-turn-on start timing, and overlapped with the APC period. That is, the state that only one light emission point is turned on is kept irrespective of incoming of the SOS detection timing. Accordingly, the amount of light incident on the SOS sensor is smaller than the light amount required to detect the SOS signal, and thus no SOS signal is generated. That is, if the rotational variation exceeds about 1.5%, the omission of the SOS signal occurs.
When the omission of the SOS signal occurs as described above, under the PLL control it is judged that the number of revolutions of the polygon motor is lowered, and the control is carried out so that the speed of the polygon motor is increased, resulting in inducing runaway of the polygon motor. Due to the runaway of the polygon motor, the operation of the image forming apparatus is stopped to reduce the productive efficiency of the apparatus.
The present invention has been implemented under these circumstances, and provides a method of controlling turn-on of a light source which enables a stable image forming operation and enables an effective scanning rate to be kept high even when the control of the rotation of a polygon motor is switched between the internal control based on an encoder in the motor and the external control based on a horizontal synchronous signal from the outside of the motor, and an image forming apparatus using the control method.
According to an aspect of the present invention, a light source turn-on control method used for an image forming apparatus in which a laser beam output from a light source scans an image carrier by rotating a rotational polygonal mirror and which has a function of switching a control operation between an internal control operation of detecting the number of revolutions of a driving motor for rotating the rotational polygonal mirror and rotating the driving motor at a predetermined number of revolutions on the basis of the detection result and an external control operation of detecting a fast scanning timing of the laser beam by a fast scanning timing detection unit disposed out of an image forming area in a fast scanning direction of the laser beam and rotating the driving motor at a predetermined number of revolutions on the basis of the detection result, is characterized in that when T1 represents the period from a start time at which turn-on of the light source is started to make the laser beam incident on the fast scanning timing detection unit a time at which the laser beam is incident on the fast scanning timing detection unit and the fast scanning timing is detected and T2 represents the time corresponding to a variation of one fast scanning time due to a rotational variation of the driving motor which is caused by the switching operation from the internal control to the external control, the turn-on of the light source to make the laser beam incident on the fast scanning timing detection unit is started so as to satisfy the relationship of T1 greater than T2 during the shift period of the switching operation from the internal control to the external control.
According to the present invention, the driving motor for rotating the rotational polygonal mirror is controlled so as to rotate at a predetermined number of revolutions (rotational speed). The control of the driving motor can be switched between the internal control based on the detection result of the number of revolutions of the driving motor and the external control based on the detection result of the fast scanning timing of the laser beam by the fast scanning timing detection unit. In order to perform the external control, it is necessarily required to turn on the light source in order to make the laser beam incident on the fast scanning timing detection unit.
Here, the turn-on starting timing to make the laser beam incident on the fast scanning timing detection unit (to detect the fast scanning timing) is set so that during the shift period of the switching operation from the internal control to the external control, the time (T1) corresponding to the period from the time at which the turn-on operation is started to make the laser beam incident on the fast scanning timing detection unit until the time at which the fast scanning timing is detected is longer than the time (T2) corresponding to the variation of the one fast scanning time due to the rotational variation of the driving motor which is caused by the switching operation (T1 greater than T2).
Accordingly, even when the number of revolutions of the driving motor is varied due to the switching operation from the internal control to the external control, the laser beam can be made incident on the fast scanning timing detection unit. That is, the fast scanning timing can be surely detected, and thus the number of revolutions of the driving motor can be returned to a desired number of revolutions and the runaway of the driving motor can be prevented.
According to another aspect of the present invention of the present invention, in the light source turn-on control method of the present invention, when T3 represents the period from the end portion of the image forming area until the detection of the fast scanning timing by the fast scanning timing detection unit, after the shift to the external control, the turn-on start timing of the light source to make the laser beam incident on the fast scanning timing detection unit may be varied so as to satisfy the relationship of T1 less than T3.
According to another aspect of the present invention, in the light source turn-on control method of the present invention, when T3 represents the period from the end portion of the image forming area until the detection of the fast scanning timing by the fast scanning timing detection unit and T4 represents the period for which the light source is turned on for the light amount control which controls the light amount of the laser beam output from the light source, after the shift to the external control, the turn-on start timing of the light source to make the laser beam incident on the fast scanning timing detection unit may be varied so as to satisfy the relationship of T1 less than (T3xe2x88x92T4).
According to another aspect of the present invention, an image forming apparatus in which a laser beam output from a light source scans an image carrier by rotating a rotational polygonal mirror to form an image, is characterized by comprising: a number of revolutions detecting unit for detecting the number of revolutions of a driving motor which rotates the rotational polygonal mirror; an internal control unit for rotating the driving motor at a desired number of revolutions on the basis of the detection result of the number of revolutions detecting unit; a fast scanning timing detection unit which is provided out of an image forming area in the fast scanning direction of the laser beam and detects a fast scanning timing of the laser beam; an external control unit for rotating the driving motor at a desired number of revolutions on the basis of the detection result by the fast scanning timing detection unit; a switching control unit for performing a switching operation between a rotational control based on the internal control unit and a rotational control based on the external control unit; and a turn-on control unit for turning on the light source earlier than a timing of incidence of the laser beam on the fast scanning timing detection unit by at least the time corresponding to a variation of one fast scanning time due to the rotational variation of the driving motor caused by the switching operation during a shift period for which the rotational control based on the internal control unit is switched to the rotational control based on the external control unit by the switching control unit.
In the light source turn-on control apparatus of the present invention, the rotation of the driving motor is controlled while switching the rotational control thereof between the rotational control based on the internal control unit (hereinafter referred to as xe2x80x9cinternal controlxe2x80x9d) and the rotational control based on the external control unit (hereinafter referred to as xe2x80x9cexternal controlxe2x80x9d, thereby rotating the driving motor at a desired number of revolutions (rotational speed). The internal control unit controls the rotation of the driving motor on the basis of the detection result of the number of revolutions detecting unit for detecting the number of revolutions of the driving motor. The external control unit controls the rotation of the driving motor on the basis of the detection result of the fast scanning timing of the laser beam by the fast scanning timing detection unit.
The turn-on control unit turns on the light source earlier than the incident timing of the laser beam during the shift period to the fast scanning timing detection unit by at least the time corresponding to the variation of one fast scanning time due to the rotational variation of the driving motor which is caused by the switching operation.
Accordingly, even when the number of revolutions of the driving motor is varied due to the switching operation from the internal control to the external control, the laser beam can be incident on the fast scanning timing detection unit. That is, the fast scanning timing can be surely detected, and thus the number of revolutions of the driving motor can be returned to a predetermined number of revolutions, thereby preventing the runaway of the driving motor.
According to another aspect of the present invention, the light source turn-on control apparatus of the present invention is further equipped with a light amount control unit for controlling the light amount of the laser beam output from the light source. After the shift to the rotational control based on the external control unit, the turn-on control unit turns on the light source out of the image forming area and before the irradiation position of the laser beam reaches the fast scanning timing detection unit, and for the light amount control based on the light amount control unit, the turn-on control unit turns on the light source out of the image forming area and during a period which is different from the turn-onon started before the irradiation position of the laser beam reaches the fast scanning timing detection unit.
According to another aspect of the present invention, in the light source turn-on control apparatus of the present invention, the light source has plural light emission points, the turn-on control unit independently turns on each of the plural light emission points, and the light amount control unit controls the light amount of the laser beam output from the light emission point concerned every light emission point.
According to another aspect of the present invention, the light source turn-on on control apparatus of the present invention is further equipped with an abnormality detecting unit for detecting rotational abnormality of the driving motor on the basis of the detection result of the number of revolutions detecting unit or the fast scanning timing detecting unit, and a ceasing unit for ceasing the detection of the rotation abnormality of the driving motor by the abnormality detecting unit during the period from the time at which the rotational control is switched from the rotational control based on the internal control unit to the rotational control based on the external control unit until the time at which the number of revolutions of the driving motor is within a predetermined range.
According to another aspect of the present invention, in the light source turn-on control apparatus of the present invention, when the rotation abnormality of the driving motor is detected by the abnormality detecting unit under the rotational control based on the external control unit, the rotational control based on the external control unit is switched to the rotational control based on the internal control unit.